The need exists in printing technology to measure the darkness of the product surface and the register relative to each other of the different colours, in order to obtain a product of commercially good quality. Web densitometers have long been used to measure the darkness of colours, and register measuring means have been used to measure the register of different colours.
The greatest shortcoming of existing register measuring methods is the large size of the register marks that are employed. The breadth of a single mark is already 6 mm, and the total space requirement of marks printed in different colours is up to 120 mm. Sufficient space cannot be found for such large marks in the folds of magazine print or equivalent advertising leaflets, nor in the margins which are trimmed off. In newspaper printing the use of a small, unobtrusive register mark is the sole possibility because from this product nothing is trimmed off at page make-up. All the same, the need of measuring as well as its required accuracy is high, as coloured illustrations are increasing in number.
Register measurement is usually based on one light source/detector pair or several light source/detector pairs disposed side by side in the direction of movement of the paper web or other printing base and monitoring each mark. Measuring of distance in the direction of travel is based on measuring the movement of the paper, and determination of lateral positioning is accomplished by examining the margin of the mark, which has been printed in a slanting position relative to the direction of travel. Furthermore, the pick-up consisting of one light source/detector pair is usually provided with a motorized or manual lateral displacement mechanism which is used at the beginning of measurement to find the marks and, during measurement, to keep the light source/detector pair at the proper point, because the paper web may become somewhat displaced.
Any lateral swings of the paper web, or variations of its tension, having an effect exceeding the size of the mark that is used will usually result in an interruption of measurement as the marks move out of the measuring range of the pick-up. For this reason marks which are as large as is feasible are nowadays used.